[OZAPRS] Newb HF questions ;)

Tue May 19 18:27:28 EST 2009

Hi Ray,

*grin* I thought I'd include as much info as possible ;)  All my RF  
work has always been on the upside of 100mhz, so I'm totally not sure  
what's actually to be expected down low :)

Re band, at this stage, I'd be happy to be able to work *any* HF  
aprs ;)  The ultimate goal is to have the rig in the car whenever we  
go away, but want to see it working in the shed first..  And with the  
hell it's causing the computer/monitors/etc so far, there's no way I'm  
puttiing it *anywhere* near my commode!

I don't really get a choice of having the AT anywhere but with the  
radio - the control cables from the radio to the AT are only 10cm  
long ;)  Plus, while very rugged, it doesn't exactly look waterproof.   
I have to wonder whether I actually should have the 4:1 balun on there  
though - I wouldn't have thought so with it being a vertical, but this  
is my first foray into HF, and the bloke at Andrews swore blind I'd  
have to have it in there...  Re the radials, I wasn't sure that they'd  
really have any use, since the antenna is only mounted 2.4m off the  
ground, but again, the bloke at Andrews swore they had to be there..   
That and the mounting to the shed isn't the strongest, and with the  
wind we get here, I thought they'd work nicely as a guys too ;)   
Reading up in my RSGB, it seems that that close to the ground, they're  
not really that much use, though it did suggest that they definitely  
shouldn't be resonant..  One's ~6m, one's ~5m, and the third is ~7m.   
Their grounding at the far end is basically clamped onto a tent peg -  
I'm wondering given that they don't do much, whether there's any point  
replacing them with 1m long stakes? Probably not?

Re the shielding stopping at the ATU, are you saying that the shield  
on the cable coming out of the ATU to the antenna is no longer at  
ground, or just that it's at ground *at* the ATU connection, but  
because of the impedance mismatch at the antenna, it gets further from  
ground as it gets closer to the antenna?  That would certainly explain  
so much RF in the shack - only the last 2 feet are outside the shack!

Given that I basically only want to work one band once I'm sorted,  
will this RF-in-the-shack problem go away if I just use a single-band  
mobile antenna on the roof of the shack?  I went the TEV-1 with an  
auto-tuner so I could play around and work any band, but if using a  
fixed-band mobile antenna will make things 'work', that's probably  
going to be muuuch easier..

And lastly, re the antenna being right above the shack causing the RF  
inside, the shack being totally metal, and earthed wouldn't stop that?

Cheers,

Damien

Damien Gardner Jnr
VK2TDG. Dip EE. GradIEAust
rendrag at rendrag.net -  http://www.rendrag.net/

On 19/05/2009, at 5:44 PM, Ray Wells wrote:

> Hi Damien,
>
> Crikey! I had to wade through lots of QRM to get to the guts of your  
> query  ;-)
> My first response is to cite an old RF engineer from many years ago  
> who advised to get the RF into the antenna and not the shack by  
> using a resonant dipole and always use a balun. However, that isn't  
> always practical, and you have a vertical.
>
> Given that you are referencing HF and aprs, can I assume you mean  
> 30m? You didn't say.
>
> Let's assume you're using 30m.
>
> An 8.5 metre vertical on 30m is about 0.287 of a wavelength, just a  
> bit over a quarter wavelength, making its impedance around the 40  
> ohm mark, with some inductive reactance. A series capacitor (use a  
> variable) would quickly enable you to tune the antenna to resonance,  
> and achieve an excellent match to 50 ohm coax. A nice low impedance  
> feed point to minimise radiation from things like the feedline (you  
> did say you had radials - are they resonant?)
>
> If you are using 40m for aprs your antenna is going to have a fairly  
> low impedance, with a high capacitive reactance, requiring series  
> inductance to achieve resonance - but that alone won't fix the  
> impedance mismatch. Also, the radials are likely to be largely  
> ineffective, unless they are resonant. However, given the low  
> feedpoint impedance that will result in higher feedpoint currents, a  
> simple array of three radials will be close to useless, albeit  
> better than nothing.
>
> You mention the ATU is bolted to the radio. That means the shielding  
> provided by the coax from the rig finishes at the ATU. If you are  
> running coax from the ATU to an antenna that is not resonant, forget  
> about the coax providing any advantage, you WILL have RF in the  
> shack. There is ONLY one place for the ATU and that's at the antenna.
>
> Of course, if the shack is close to the antenna, your shack  
> equipment will likely be affected by radiation from the antenna no  
> matter how well the antenna is fed and matched. My HF antennas are  
> single-band, balun fed dipoles up 45', and 150 feet from the shack,  
> with the coaxial cables - most FHJ4-50 - buried in ducts. I still  
> see occasional RF problems, and the CRO goes berserk.
>
> Ray vk2tv
>
> Damien Gardner Jnr wrote:
>> Howdy Folks,
>>
>> So I've *finally* unpacked all my gear from their boxes, fixed a  
>> few faults (seems a dry joint on the output DIN socket was why the  
>> old MFJ running the vk1 igate went silent), and thought I was ready  
>> to play APRS on HF..  But it seems not..  After doing a lot of  
>> playing about with the help of an immensely more RF-knowledgable  
>> colleague, I've come to realise that I live in a pretty horrid area  
>> to be trying to do anything RF-related in, but I'm going to try and  
>> struggle through that..
>>
>> Backstory - when we shifted here from 10km down the road, we had to  
>> replace our 900mhz cordless phone with a 5.8ghz DECT setup, as the  
>> mains hum on the old one was pretty horrid..  When we got a baby  
>> monitor earlier this year (yep, I added to the pool of hams with  
>> girl children.. seriously, does ANYONE in our hobby have boys?!?),  
>> we had to replace that with a DECT unit, as that was absolutely  
>> horrid..  and then that DECT unit had to be replaced with one from  
>> a different manufacturer, as even IT had 50hz mains hum..  I  
>> actually spoke to the bloke that designed the DECT one we had  
>> problems with, and he was pretty concerned, as he figured the only  
>> place it'd be getting in was actual mains proximity to the  
>> receiver.. - but it was doing it in the carpark at the local  
>> shopping centre!  But stopped 10km down the road..  APRS on my D7  
>> is completely deaf, even though RAG hears me fine, and my FT8100R  
>> with a 9db colinear on the roof of the shed can hear it and RAG  
>> fine - but get more than a block or two away with the D7, and the  
>> 8100 no longer hears it even at 5W output!?
>>
>> Anyway, back to my problems, and questions. If there's a list more  
>> appropriate, PLEASE point me at it ;)  This is just the only ham- 
>> related list I'm on (apart from the APRS admin list..)
>>
>> So I'm finally setup - the FT8100R with a 2/70 9/6db mobile antenna  
>> (used to be on my luxie) mounted above the roof of the shed (http://photos-a.ak.fbcdn.net/photos-ak-sf2p/v646/250/124/506862762/n506862762_1735424_1072471.jpg 
>> ).  And an FT-897 with an LDG AT-897 feeding RG-58U (couldn't see  
>> any point running LMR-400 for an 8-meter run on HF..?) to a TEV-1  
>> (8.5m aluminium vertical ) via a 4:1 balun (http://photos-f.ak.fbcdn.net/photos-ak-sf2p/v646/250/124/506862762/n506862762_1735429_1155875.jpg 
>> ). (There's been three radials added since that shot - 120 degrees  
>> apart, at around 40 degrees down, grounded at the ends, and bonded  
>> to the 'radials' point on the antenna end)
>> The pole the TEV-1 is mounted on is rammed about 20cm into the  
>> ground at the base, and is touching the shed roof about 50cm below  
>> the base of the antenna - not sure on whether I should be  
>> insulating that point or earthing it, or leaving it as is?
>>
>> The shed is thin steel sheet, and is earthed both to the AC earth,  
>> and via a 1m steel rod driven 90cm into the ground (seriously, have  
>> you seen the cost of copper clad earth rods recently?!?), with 5mm  
>> steel cable (I have *so* much of the clear plastic-covered 5mm  
>> steel cable left over from old microwave link installs, and it fits  
>> perfectly into 5mm crimp connectors ;) ) to the frame of the shed.   
>> There's also a run of cable bolted from the frame of the shed to  
>> the -ve terminal of the 60A 12V supply, and to the gnd connection  
>> on the back of the FT-897.
>>
>> So anyway, apart from having S8-9 noise below 18Mhz, which I'm just  
>> going to try to work around, I'm fairly sure there's something not  
>> right in my setup.. - Any time I hit TX below 20Mz, my USB mouse  
>> turns itself off (has to unplugged and re-plugged), my two LCD  
>> monitors go wavey (like old crt's would when you hit tx with a  
>> handheld near them..), and all the serial ports into the pc in the  
>> shed start having random crap appear on them..  Since I'm seeing  
>> the meter movements in my SWR meters (neither of which are plugged  
>> into anything at the moment, though are powered up..) are coming up  
>> to about 1/2 a watt, I got to and pluged a probe into the cro, and  
>> attached a metre of wire to the tip.. -  I'm seeing 1V p-p of  
>> signal (tx'ing on am, a nice clean sinewave at the tx frequency)  
>> inside the shed, which I wouldn't normally expect, especially with  
>> the shed frame (and it's a completely steel shed..) being grounded?  
>> I would have thought that with the case of the 897 grounded, an the  
>> antenna tuner bolted to the 897, there shouldn't be any radio  
>> signal anywhere but the coax, and that shouldn't be radiating??   
>> Plugging the 2m/70cm antenna's cable straight into the cro, I see  
>> 5v P-P, but that's outside the shed, so I'd kinda expect that..
>>
>> SO.. After that extremely long blurb, does anyone have any  
>> thoughts? *should* I be seeing that much RF inside the shed?  If  
>> not, where should I look next?  Is it worth reaming a hole in the  
>> shed wall, and make up a run of LMR-400?  I wouldn't have thought  
>> that the RG58 would be an issue, but maybe?  Or should the majority  
>> of the coax run be outside the shed, rather than inside? (again  
>> though, I wouldn't have thought the coax would be radiating?)
>>
>> Thanks!
>>
>> Damien
>>
>> Damien Gardner Jnr
>> VK2TDG. Dip EE. GradIEAust
>> rendrag at rendrag.net -  http://www.rendrag.net/
>> -- 
>> We rode on the winds of the rising storm,
>> We ran to the sounds of thunder.
>> We danced among the lightning bolts,
>> and tore the world asunder
>>
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